Arc discharge lamps such as fluorescent lamps which employ a phosphor within a discharge chamber, which chamber also contains an ionizable medium together with mercury vapor, suffer from a gradually decreasing light output as they age. Various factors contribute to the drop-off in light output during operation, and some of these may be caused by deposits of impurities from the cathode; the formation of various compounds of mercury; changes in the phosphor itself; and changes in the glass envelope, particularly where it may be subject to ultraviolet radiation. The ability of such lamps to resist drop-off in light output is generally termed lumen maintenance, and it is measured as the ratio of light output at a given life span compared to an initial light output and expressed as a percentage. Since the light output of a new lamp is apt to vary considerably until it has been in operation for some time, it is usual to start lumen maintenance measurements from some time other than time zero.
While decreasing light output with time is an occurrence for all fluorescent lamps, it is much more of a problem for high output and very high output lamps than it is for normally loaded lamps.
Although all of the conditions enumerated above can be present to a greater or lesser degree in acting to reduce light output, it is presently believed that the formation of mercury compounds, particularly on the surface of the phosphor, is one of the primary factors.
It is believed that these mercury compounds form an ultraviolet radiation absorbing film which prevents the phosphor from receiving sufficient exciting radiation from the mercury discharge to achieve maximum light output.
Various uses of alumina within such lamps have been proposed to alleviate this condition. For example, U.S. Pat. Nos. 4,079,288 and 4,058,639, as well as others, discuss employing a layer of alumina on the interior of the bulb wall and applying phosphor thereof.
U.S. Pat. No. 3,886,396 teaches the application of a thin, porous, discontinuous layer of alumina being applied over the phoshor layer. While all of these techniques provide some benefit, it is believed that further increases in lumen maintenance are desirable.